Floating roof mixing tank



Dec. 27, 1966 G. N. DAVIS ETAL 3,294,372

FLOATING ROOF MIXING TANK Filed Dec. 14, 1964 5 Sheets-Sheet 1 FIG-Z INVENTOR5 650E615 M DAV/5 M4)( 5. SOUT/IW/CK Bea 1966 G. N. DAVIS ETAL FLOATING ROOF MIXING TANK 5 Sheets-Sheet 2 Filed Dec. 14, 1964 INVENTORS GEORGE A/. DAV/5 MAX 5 SOUTH/406K ite This invention relates generally to floating roof storage tanks, and more particularly to floating roof tanks having outwardly protruding housings forming bays in the tank shell to accommodate the recessed mounting of mixer propellers.

When a volatile liquid is stored in a conventional cone roof tank, a portion of the liquid vaporizes and fills the vapor space between the surface of the stored liquid and the tank roof. This vapor is displaced to the atmosphere on thermal expansion, or when the tank is filled, thus causing the loss of Valuable commodity. Further, when the tank contents are cooled, or the tank is emptied, air is drawn into the vapor space. If a combustible commodity is being stored, as is frequently the case, the mixture of combustible vapor and oxygen from the air can be explosive, thus creating a serious fire hazard. The problems of breathing losses and explosive Vapor spaces are greatly minimized with floating roof tanks, as the tank construction is such that the vapor space between the surface of the liquid and the tank roof is eliminated. In these tanks, the roof normally floats on top of the stored liquid by virtue of its own buoyancy, raising or lowering as the liquid is pumped into or removed from the tank. For these reasons, floating roof tanks are widely employed in the petroleum and chemical industries for the storage of volatile liquids.

It is common practice to employ floating roof tan-ks for the blending of two or more components, as well as for the storage of liquid commodities. Mixing of the blend components is frequently accomplished by propeller mixers mounted on shafts extending through the tank shell into the interior of the tank. Such mixers can also prevent separation of the stored contents of the tank by settling. In any case, it is usually desirable to mount these mixers at some distance above the bottom of the tank to minimize the amount of water, sludge and tank bottoms mixed with the blend components. The floating roof is typically provided with a plurality of adjustable legs, or supports, to limit the travel of the roof in the downward direction. In the case of a tank equipped with mixers, the legs may be adjusted so that the roof is supported above the mixer propellers to prevent damage to the propellers, shafts and roof. Since lowering the level of the liquid in the tank to a point where the roof no longer floats is undesirable, as the principal advantages of a floating roof tank are thereby lost, suflicient commodity must at all times be retained in the tank to cause the roof to float above the mixer propellers. Thus, not only is considerable commodity contained in unavailable inventory, but a substantial proportion of the tank volume is not available for use. For example, in a 150,000 barrel capacity tank, as much as 15,000 barrels of liquid must be maintained in the tank to keep the roof floating above the mixer propellers. Floating roof tanks are relatively expensive to construct and maintain, thus this unavailable storage capacity represents a large investment in both unused tank capacity and commodity.

The prior art has attempted to reduce the unavailable heel by installing wells in the tank roof surrounding the mixers, so that the roof may be lowered to a point below the mixer propellers, the mixers being housed within these wells when the roof is in the low position. However, the installation of such recesses, or wells, in the roof is untates Patent desirable as vapor spaces are created in which explosive mixtures can accumulate, and even with such wells the extent to which the roof may be lowered is limited.

A principal object of this invention is to provide increased useful storage capacity in a floating roof tank equipped with propeller mixers.

Another object of this invention is to provide an improved mounting for side-entering propeller mixers installed in a floating roof storage tank.

Still another object of this invention is to provide a mounting for propeller mixers installed in a floating roof tank that will permit unimpeded vertical travel of the tank roof within the tank.

Other objects and advantages of my invention will be apparent to those skilled in the art from the description thereof which follows.

We have found that the foregoing objects and their attendant advantages can be realized by mounting propeller mixers installed on floating roof tanks in mixer bays recessed into the tank shell so that the innermost extremity of the mixer assembly lies outside the principal radius of the tank, thereby allowing the floating roof unimpeded vertical travel past the mixer as the level of liquid in the tank is raised and lowered. The mixer bay of this invention is formed by a protuberance, or housing, projecting outwardly from the tank shell and enclosing an internal recess in the shell open to the interior of the tank. This housing can be of hemispherical, extended hemispherical, rectangular, or other shape, of sufficient size to contain at least one mixer propeller, and can be a formed head adjoined to the tank shell by welding, or other means, so that it becomes an integral part thereof. A nozzle can be located in the housing to permit entry of the propeller shaft through the wall thereof. Any type of side-entering, propeller type tank mixer may be employed with the mixer bay of my invention, the invention lying in the recessed mixer bay formed by the outwardly protuberant shell section and not in the particular mixer employed therewith.

Conventionally, the mixer assembly will comprise an electric motor drive and support means located outside of the housing and adapted to drive a propeller positioned Within the aforesaid recessed mixer bay. The propeller is mounted on a rotatable shaft which extends through the housing wall, the shaft being sealed by a packed, or mechanically sealed joint at the point of its entry into the tank. Conveniently, the mixer assembly is bolted to a nozzle flange aflixed to the housing. In this installation, the mixer propeller is located in the liquid contents of the tank, yet is positioned entirely within the recessed mixer bay and does not extend into the tank past the non-protruding portion of the tank shell. Thus, mixing of the tank contents can be effected when the tank is filled sufficiently that the roof floats above the mixer bay, yet the roof is free to travel past the mixer bay as the contents of the tank are removed and the liquid level therein is lowered, without damage to the mixer or to the floating roof.

The mixer bay of this invention can best be understood by reference to the drawings, of which:

FIGURE 1 is a cross-sectional schematic representation of a floating roof tank showing a propeller mixer mounted in a recessed mixer bay formed by an outwardly protruding housing.

FIGURE 2 is an elevation view of a section of a tank shell showing a mixer housing installation.

FIGURE 3 is a plan view of the mixer housing of FIGURE 2.

FIGURE 4 is a side elevation view in cross-section, taken along the line 11 of FIGURE 2.

FIGURE 5 is an elevation view of a section of a tank shell showing another embodiment of mixer housing having an extended hemispherical shape and including an internal flow diverter.

FIGURE 6 is a plan view in cross-section showing the extended hemispherical mixer bay and internal flow diverter of FIGURE 5, taken along the line 11 of FIG- URE 5.

FIGURE 7 is a cross-sectional view of the internal flow diverter along the line 22 of FIGURE 6.

FIGURE 8 is a detail of the flow diverter installation within the mixer housing.

The installation of the mixer housing of this invention in a typical floating roof tank is schematically illustrated in FIGURE 1. Referring now to FIGURE 1, the tank comprises a substantially cylindrical shell 10, open at the top and enclosed at the bottom by attachment to floor 11 by a liquid-tight joint around its periphery. Shell is strengthened by top angle 12 attached to the shell and extending around the shell at its top periphery. Additional strength is supplied to shell 10 by wind girder 13 located near the top, below top angle 12, and similarly attached to the shell.

The floating roof may be of any conventional type, i.e., pan, pontoon, or double-deck type. For illustrative purposes, a double-deck type roof is shown in FIGURE 1. Floating roof 14 is comprised of top deck 15 and bottom deck 16, both substantially covering the cross-section of the tank and the liquid contents thereof. The two decks are spaced apart by walls 17 extending around the periphery of decks 15 and 16, and by interior bulkheads, not shown, which divide the roof section into separate liquid-tight compartments. With this construction, a buoyant structure is formed which will float on the surface of the liquid, and which is separately compartmented so that a leak in any individual compartment will not cause the roof to sink. Roof shoes 18 extend around the periphery of roof 14 and are attached thereto and supported by springs and anchor means not shown. These plates contact the interior surface of cylindrical shell 10 and slidably bear against the shell as the roof rises and lowers with changing liquid level. The annular space between the roof wall plate 17 and roof shoe 18 is covered by a fabric membrane 19 which prevents vaporization from the otherwise exposed liquid surface about the periphery of the roof decks. Thus the area inside of shell 10 is effectively covered by the roof decks and the fabric seal. Alternatively, roof shoes 18 may be replaced by a resilient seal, such as a synthetic rubber tubular seal, which functions to prevent vaporization of the liquid contents of the tank from around the periphery of roof 14.

Recessed mixer housing 20 is a protruding section of tank shell 10 adapted to house a propeller type mechanical mixer in a position such that the propeller does not extend into the tank to a point which would interfere with the vertical travel of floating roof 14 as liquid is pumped into and removed from the tank. The mixer of this illustration comprises electric motor drive 21 installed on the exterior of the tank adjacent mixer housing 20. Mixer propeller 23 is mounted on rotatable, motor-driven shaft 22, and positioned wholly within the recessed mixer bay formed by housing 20. Shaft 22 enters the tank through nozzle 24, and can be optionally sealed by packing glands or mechanical seals, not shown. Although an electric motor is shown in FIGURE 1 for illustrative purposes, any drive means external to the tank may be utilized as a means of driving propeller 23. The mixer assembly is supported by rods 26 attached to clips 25, only one rod and clip assembly being shown in the drawing.

The shell of a floating roof tank is typically constructed of a number of steel plates, rolled to obtain a vertical cylindrical configuration, and fabricated into an integral shell by welding, riveting, bolting, or other means. The protruding mixer housing may be formed by shaping one or more of the shell plates to achieve the desired configuration. However, a preferred method of fabrication is to form the mixer housing as an integral unit, either by shaping a steel plate to the desired configuration, or by assembling a number of shaped pieces to attain the desired structure, and then fixedly attaching the mixer housing to the tank shell. Installation of the mixer housing may be made during the original construction of the tank, or by simple modification of an existing tank. Details of a preferred hemispherically shaped mixer housing fabricated as an integral unit, and then attached to a tank shell are depicted in FIGURES 2, 3, and 4.

Referring now to FIGURES 2 and 3, hemispherically shaped housing 30 is installed on a cylindrical tank shell, a section of which is shown at 31. Shell 31 is attached to floor plate 32 by welding to achieve a liquid-tight joint. Housing 30 is installed in a cutout in shell 31, especially made for this purpose, and attached to shell 31 by welding. The joint between housing 30 and shell 31 is strengthened by means of reinforcing pad 33, constructed of a flat plate rolled to fit the contour of shell 31, and cut to fit around housing 30. Reinforcing pad 33 is welded to shell 31 at its outer periphery and to housing 30 about the periphery of the cutout. A small hole, 34, is drilled through the pad to provide a vent for the sealed pocket between the exterior surface of shell 31 and the interior surface of pad 33, thereby providing an indication of any leak in the interior welds.

Housing 30 may be of any size so long as the recessed mixer bay formed thereby is capable of housing a mixer propeller without substantial restriction of the flow of liquid thereto. Although excessively large mixer housings are to be avoided as unduly costly, the housing should be sized to provide adequate suction to the mixing propeller. In general, suitable housings are from about 3 feet to about 8 feet in diameter, and are positioned on the tank so that the center of the mixer bay formed thereby is at a distance of between about 2 feet and about 6 feet from the bottom of the tank. Particularly with larger diameter mixer housings, it is not necessary that a true hemispherical, or symmetrical, shape be maintained, and flattened, or dished shapes are preferred. In such configurations, the depth of the recessed mixer bay will be less than the radius of the bay at its open face. Where a flattened housing is not used and the mixer bay is relatively deep, the propeller should be located close to the open face of the bay.

Nozzle 35 is provided in housing 30 as a means for entry of the rotatable propeller shaft into the tank. Nozzle 35 is typically constructed of a short piece of pipe having a standard flange 36 welded thereto. A flanged nozzle is preferred inasmuch as the mixer can then be attached directly to the tank by stud bolts inserted through holes 37 about the periphery of flange 36. Nozzle 35 may be relatively small, from about 3 inches to about 12 inches in diameter, capable of providing entry for the propeller shaft, or it may be sufliciently large, from about 18 inches to about 30 inches in diameter, to permit removal of the mixer propeller therethrough. Nozzle 35 may be installed at the center of mixer bay 30, or as illustrated in FIGURE 2, the nozzle may be installed below the center line of the mixer housing. Suoh installation provides more clearance around the top of the propeller, thereby affording increased flow area for the disproportionally large volume of liquid flowing to the propeller around its top periphery. Nozzle 35 can be installed from about zero to about 12 inches, or more, below the center line of the mixer bay, depending on the clearance between the bottom of the propeller and housing 30. Installation of a mixing propeller in a recessed mixer bay tends to decrease suction to the propeller, thereby restricting the volume of liquid moved by the mixer and decreasing the amount of mixing accomplished This restriction of the propeller suction can be minimized and essentially eliminated, by providing adequate clear ance around the propeller, particularly around the top periphery thereof.

Tank mixers are typically installed at a slight horizontal angle to impart circumferential movement to the liquid, thereby affording improved flow patterns within the liquid contents of the tank. In FIGURE 3 it is seen that nozzle 35 is installed on mixer housing 30 at an angle from the center of the tank. This angular offset may be from about degrees to about 30 degrees, many mixers being conventionally installed at an angle of about 10'degrees from the center of the tank.

The details of attachment of housing 30 to shell 31 are shown in FIGURE 4. Field erection can be simplified by forming housing 30 with a straight flange 46 about the periphery of its open face, flange 411 being a straight extension of the curved section of the housing and preferably not exceeding a width of about 6 inches. On installation, housing 30 is inserted into the tank through the cutout in shell 31. The protruding portion of straight flange 40, shown dotted in FIGURE 4, is trimmed off so that housing 30 does not project into the tank past the interior surface of shell 31. Preferably, the protruding flange is trimmed so that it extends into the tank past the exterior surface of shell 31 a distance of about one half of the shell thickness, thereby affording a superior surface for welding.

The cutout in shell 31 should be made to permit insertion of housing 30 without excessive clearance. A small clearance between housing 30 and shell 31, usually about /2 inch, but not exceeding about 1 inch on all sides, is desirable to accommodate welding of housing 30 to shell 31. The cutout in reinforcing pad 33 should be of smaller diameter than the cutout in shell 31, thereby affording less clearance around housing 30. A clearance of -inch, or less, between housing 30 and pad 33 is satisfactory in most installations.

Housing 30 is attached to shell 31 by means of continuous fillet weld 41 about the periphery of the cutout in shell 31, which weld integrally joins housing 36 and reinforcing pad 33 to shell 31. A small fillet weld 42 is made about the periphery of housing 30, external to the tank. Reinforcing pad 33 is further attached to shell 31 about its outer periphery by means of continuous fillet weld 43. Nozzle 35 is similarly installed in a cutout in housing 30 and fixedly attached thereto by fillet welds 44 and 45 on the interior and exterior of the tank, respectively. Nozzle 35 is preferably triinmed to project into the tank sufiiciently to accommodate interior weld 44, a projection of less than about 1 inch usually being adequate.

Although the preferred method of attachment of the mixer housing to the tank shell is by welding, as described above, any suitable method of attachment may be employed. For example, housing 30 my be flanged and attached to shell 31 by bolting to a companion flange installed thereupon. Thus the entire mixer housing may be removed from the tank, leaving a flanged manhole for access to the tank.

As previously mentioned, housing 30 may be of any convenient shape. Although the hemispherical, or dished housing circular in cross-section, is often preferred, a configuration illustrated in FIGURE affords improved propeller suction, thereby yielding improved mixing. This embodiment is similar to the aforementioned hemispherical housing, except that it is horizontally extended thereby defining a recessed box in the tank shell of generally rounded structure having a horizontal axis which is longer than the vertical axis of the housing. Referring now to FIGURE 5, housing 50 is shown installed in a tank shell, a section of which is depicted at 51. Again, shell 51 is attached to floor plate 52. Reinforcing pad 53, with vent hole 54, is provided to increase the strength of the peripheral joint. Nozzle 55, with flange face 56 having threaded bolt holes 57, is provided for entry of the mixer shaft. A flanged nozzle similar to that illustrated in FIGURE 2 may be installed in housing 50 in place of the pad type nozzle depicted in this illustration. Again nozzle 55 can be installed at a horizontal angle to impart circumferential movement to the liquid contents of the tank, and can also be located below the center of housing 50. Housing 50 may be formed so that its depth is no greater than required to contain the propeller assembly without restricting vertical movement of the tank roof. Welding details are similar to those illustrated in FIGURE 4 with reference to the hemispherical configuration.

Mixer housing 50 illustrated in FIGURES 5 and 6 has an internal flow diverter 60 installed therewithin to improve suction to the mixer propeller. Installation of the propeller in a recessed mixer hay increases the tendency of the fluid to c'avitate about the periphery of the propeller blade as fluid is flowing into the housing around the outer periphery of the propeller and away from the face of the propeller. The fluid flow is somewhat restricted by the mixer housing causing the influent to the propeller and eflluent thereform to flow in substantially opposite directions. Substantial turbulence can exist at the common boundary of these oppositely flowing fluid masses which can produce undesirable cavitation of the fluid. Experience has indicated improved performance where the fluid enters from the rear of the propeller in a direction axial to the propeller shaft without a high component of velocity in a direction normal thereto, and is discharged from the face of the propeller in substantially the same axial direction. Installation of the flow diverter of this invention decreases this turbulence and causes the fluid to flow into the propeller substantially axially to the propeller shaft. Propeller 58 is illustrated installed in flow diverter 60.

As illustrated in FIGURE 7, flow diverter 60 comprises an outer ring 61 forming an outer housing for the mixer propeller. Ring 61 should have an inside diameter between about 1 and about 6 inches larger than the outer diameter of the propeller blade, and is installed coaxially therewith. Ring 61 can be fabricated from a band of metal so as to form a circular ring having a [width of between about 4 and about 12 inches. Preferably ring 61 is fabricated with an inside diameter about 3 inches larger than the diameter of the propeller. Ring 61 is positioned within the mixer housing and surrounds the mixer propeller, being supported therein by means of a plurality of straightening vanes 62 which are flat plates cut to fit the curvature of housing 50. Although any number of such vanes may be employed, an arrangement such as illustrated in FIGURE 7 employing four vanes is preferred. These vanes may be oriented about ring 61 with an angular displacement of between about 15 and about 60 degrees from the vertical axis, a displacement of between about 25 and about 35 degrees being preferred. Ring 61 and straightening vanes 62 are installed within housing 50 as illustrated in FIGURE 8. Straightening vane 62 is attached to ring '61 and to housing 50 conveniently by welding or other means of attachment. Although flow diverter 60 is illustrated as installed within extended hemispherical housing 50, it is equally adaptable to installation in housing of other configurations.

While hemispherical, dished circular and extended hemispherically shaped mixer housings have been particularly described, it is within the scope of our invention to employ a mixer housing of any configuration adapted to contain a mixer propeller in a recessed bay in the tank shell, and to employ any suitable means of fabrication and attachment of such mixer housing to the tank shell. It is further within the scope of our invention to adapt any mixing means to mounting in our recessed mixer bays. These and various other modifications will be apparent to those skilled in the art, Without departing from the scope and spirit of our invention as defined by the following claims.

We claim:

1. A floating roof tank for the storage and blending of liquids comprising:

a substantially cylindrical vertical shell enclosed at the bottom by a floor and having an outwardly protruding section spaced from said floor so as to form a recess in said shell open to the interior of said tank;

a vertically movable floating roof within said shell substantially covering the cross-sectional area thereof and adapted to float on the surface of said liquids; and

mixing means for mixing said liquids mounted within said recess in said shell.

2. In a floating roof tank for the blending and storage of liquids comprising a substantially cylindrical vertical shell, a floor enclosing the bottom of said shell, a vertically movable roof within said shell adapted to float on the surface of said liquids, and a mixer propeller mounted on a rotatable shaft extending into said tank through said shell, the improvement in combination therewith which comprises an outward protrusion of said shell spaced from said floor forming a recessed mixer bay open to the interior of said tank, said propeller being positioned in said mixer bay outside of the vertical path traveled by said roof as the level of said liquid in said tank is raised and lowered.

3. In combination:

a floating roof tank comprising a substantially cylindrical shell enclosed at the bottom by a floor and having a floating roof within said shell adapted to float on the surface of said liquid contained in said tank;

a recessed mixer bay open to the interior of said tank and formed by an outwardly protruding section of said shell spaced from said floor;

a rotatable shaft passing through said protruding housing into said recessed mixer bay; and

a mixing propeller mounted on said rotatable shaft and located within said recessed mixer bay so as to afford unimpeded vertical travel for said floating roof as the level of liquid contained in said tank is raised and lowered.

4. The combination of claim 3 including a flo w diverter located within said mixer bay formed by said protruding housing, said flow diverter comprising a cylindrical ring around the periphery of said mixing propeller and positioned coaxially therewith, said ring being supported within said housing by a plurality of flat straightening vanes attached to said ring and to said housing.

5. A floating roof tank for the blending and storage of liquids comprising:

a substantially cylindrical vertical shell;

Ia floating roof within said shell substantially covering the cross-section thereof and adapted to float on the surface of said liquids contained in said tank, said roof being vertically movable "as the level of said liquid in said tank is raised and lowered;

an outwardly protruding housing fixedly attached to said shell to form a recessed mixer bay open to the interior of said tank and extending outwardly from said shell, said mixer bay being otherwise enclosed by said housing;

a reinforcing pad comprising a plate contoured to fit against said shell and formed to fit around said outwardly protruding housing, said pad attached to the exterior of said shell and to said housing;

a flanged nozzle fixedly attached to said housing and communicating with the interior thereof;

a mixer mounted outside of said tank with a rotatable, sealed shaft projecting through said flanged nozzle into said mixer bay; and

a propeller attached to said shaft and contained within said recessed mixer bay.

6. The apparatus of claim 5 wherein said housing is of a hemispherical shape.

7. The apparatus of claim 5 wherein said housing has a dished shape with a circular cross-section and a radius at the open face of said dish exceeding the depth thereof.

8. The apparatus of claim 5 wherein said housing has an extended, generally hemispherical cross-section, the horizontal axis of the housing being longer than the vertical axis thereof.

9. The apparatus of claim 5 wherein said flanged nozzle is located at an elevation below the center line of said housing.

10. The apparatus of claim 5 wherein said flanged nozzle is oriented at an angle of from about 0 degrees to about 30 degrees from the center of said tank.

11. The apparatus of claim 5 wherein said housing and said reinforcing pad are attached to said shell by welding.

12. The apparatus of claim 5 including a flow diverter located within said recessed mixer bay formed by said protruding housing, said flow diverter comprising a cylindrical ring around the periphery of said propeller and positioned coaxially therewith, said ring being supported within said housing by a plurality of flat straightening vanes attached to said ring and to said housing.

References Cited by the Examiner UNITED STATES PATENTS 2,577,797 12/1951 Moyer. 3,022,052 2/1962 Smith 259- 3,051,457 8/1962 Rice 259121 3,112,634 12/1963 Klykken 259-95 WALTER A. SCHEEL, Primary Examiner.

R. W. JENKINS, Assistant Examiner. 

1. A FLOATING ROOF TANK FOR THE STORAGE AND BLENDING OF LIQUIDS COMPRISING: A SUBSTANTIALLY CYLINDRICAL VERTICAL SHELL ENCLOSED AT THE BOTTOM BY A FLOOR AND HAVING AN OUTWARDLY PROTRUDING SECTON SPACED FROM SAID FLOOR SO AS TO FORM A RECESS IN SAID SHELL OPEN TO THE INTERIOR OF SAID TANK; A VERTICALLY MOVABLE FLOATING ROOF WITHIN SAID SHELL SUBSTANTIALLY COVERING THE CROSS-SECTIONAL AREA THEREOF AND ADAPTED TO FLOAT ON THE SURFACE OF SAID LIQUIDS; AND MIXING MEANS FOR MIXING SAID LIQUIDS MOUNTED WITHIN SAID RECESS IN SAID SHELL. 